Hereinafter, the term “layer system” is defined as follows: a layer system consists of one or more layers, wherein the respective layers can be individual layers or mixed layers. The layer system can thus be an arbitrary combination of individual and mixed layers.
Hereinafter, the term “transport layer system” is defined as follows: a transport layer system is a layer system which transports preferably one type of charge carrier (electrons: n-type transport layer system; holes: p-type transport layer system). In a solar cell, through absorption of light, a transport layer system contributes less than 5% to the short-circuit photocurrent, preferably less than 2%.
Furthermore, the term “different transport layer systems” is defined as follows: two transport layer systems are different if at least one material is contained only in one of the two transport layer systems. If said material is a dopant, then the transport layer systems are different when each transport layer system contains a different dopant.
The term “charge carrier type” is defined as follows: a charge carrier type relates to electrons and holes. In this sense, two transport layer systems are of the same charge carrier type if they both preferably conduct electrons or both preferably conduct holes.
The term “transparent” is defined as a material or a material film or a transport layer system is transparent if at least one of the following conditions is applicable:
The energetic separation between LUMO and HOMO of the material or of the material film or of the transport layer system is >2.5 eV, preferably >3.0 eV.
A 50 nm thick film of the material or of the transport layer system has a transparency of >70% in the wavelength range of 400 nm to 900 nm, preferably >80% or >90%, very preferably >95%.
The material or the material film or the transport layer system has an extinction coefficient ε which does not exceed the value of 0.5×105 cm−1 in the wavelength range of between 450 nm and 800 nm, and/or the material or the material film or the transport layer system has an absorption index k which does not exceed the value 0.1 in the wavelength range of between 450 nm and 800 nm (another term for k is also optical constant: the two optical constants are usually designated by n and k).
The material or the material film or the transport layer system has a larger optical band gap than the photoactive layer system (within the meaning of DE 10 2004 014 046).
Transparent organic materials are also designated as wide-gap materials in the literature.
The terms “HOMO” and “LUMO” are understood as highest occupied molecular orbital and lowest unoccupied molecular orbital, as usual in chemistry. The term in this case relates both to individual molecules and to solids or material films. In this case, the energy positions of HOMO and LUMO can be determined, as known to the person skilled in the art, e.g., by means of cyclic voltammetry (CV) or ultraviolet photoelectron spectroscopy (ultraviolet photon spectroscopy UPS).
The term “transport energy level position” is defined as follows: the transport energy level position of a transport layer system is the energetic position of the HOMO if a p-type transport layer system is involved, and is the energetic position of the LUMO if an n-type transport layer system is involved.
Furthermore, the term “energetically adapted” is defined as follows: a transport layer system which preferably conducts electrons (n-type conductor) is energetically adapted to a photoactive layer system if the energy level of the LUMO of the transport layer system differs by less than 0.5 eV from the energy level of the LUMO of the acceptor material of the photoactive layer system. In this case, the energy level of the LUMO of the transport layer system can be either a maximum of 0.5 eV above the energy level of the LUMO of the acceptor material of the photoactive layer system or a maximum of 0.5 eV below. If the photoactive layer system contains a plurality of acceptors, then that acceptor material which has the energetically lowest LUMO is decisive. Preferably, the energy level positions of the LUMOs can also differ only by 0.3 eV, particularly preferably also only by 0.2 eV or only by 0.1 eV, or they can be virtually identical.
Analogously, a transport layer system which preferably conducts holes (p-type conductor) is energetically adapted to a photoactive layer system if the energy level of the HOMO of the transport layer system differs by less than 0.5 eV from the energy level of the HOMO of the donor material of the photoactive layer system. In this case, the energy level of the HOMO of the transport layer system can be either a maximum of 0.5 eV above the energy level of the HOMO of the donor material of the photoactive layer system or a maximum of 0.5 eV below. If the photoactive layer system contains a plurality of donors, then that donor material which has the energetically highest HOMO is decisive. Preferably, the energy level positions of the HOMOs can also differ only by 0.3 eV, very preferably also only by 0.2 eV or only by 0.1 eV, or can be virtually or exactly identical.
Possibilities for adapting the LUMO levels or the HOMO levels are known to the person skilled in the art, wherein very many organic materials having different positions of the energy levels of the HOMOs and LUMOs are known. The adaptation is therefore effected in such a way that a material which has the desired position of the energy levels of the HOMO and LUMO is selected and used. Furthermore, e.g., by incorporating electron-attracting or electron-repelling groups, the HOMO and LUMO levels of organic materials can be decreased or increased and a material can thus be adapted in accordance with the requirements.
In the context of this application, the term “readily dopable” is defined as follows: a hole transport material (p-type transport material) or a hole transport layer system (p-type transport layer system) is designated as readily dopable if the energy level of its HOMO is greater than or equal to −5.5 eV. The term “greater” (or else “higher”) here relates to the numerical value, i.e., −5.4 eV is greater (higher) than −5.5 eV. Preferably, the energy level of the HOMO is in the range of −5.2 eV to −4.9 eV. Analogously, an electron transport material (n-type transport material) or an electron transport layer system (n-type transport layer system) is designated as readily dopable if the energy level of the LUMO is less than or equal to −3.0 eV. The term “less” (or else “lower”) here relates to the numerical value, i.e., −3.1 eV is less (lower) than −3.0 eV. Preferably, the energy level of the LUMO is in the range of −3.5 eV to −4.5 eV.
Since the demonstration of the first organic solar cell having an efficiency in the percent range by Tang, et al., 1986 [C. W. Tang, et al., Appl. Phys. Lett. 48, 183 (1986)], organic materials have been investigated intensively for various electronic and optoelectronic components. Organic solar cells consist of a sequence of thin layers (typically 1 nm to 1 μm) composed of organic materials, which are preferably applied by vapor deposition in a vacuum or by spin-coating from a solution. The electrical contact-connection can be effected by metal layers, transparent conductive oxides (TCOs) and/or transparent conductive polymers (PEDOT-PSS, PANI).
A solar cell converts light energy into electrical energy. In this case, the term photoactive likewise denotes the conversion of light energy into electrical energy. In contrast to inorganic solar cells, in organic solar cells the light does not directly generate free charge carriers, rather excitons initially form, that is to say electrically neutral excitation states (bound electron-hole pairs). It is only in a second step that these excitons are separated into free charge carriers which then contribute to the electric current flow.
The advantage of such organic-based components over the conventional inorganic-based components (semiconductors such as silicon, gallium arsenide) are the in some instances extremely high optical absorption coefficients (up to 2×105 cm−1), thus affording the possibility of producing very thin solar cells with little outlay in terms of material and energy. Further technological aspects include the low costs, the possibility of producing flexible large-area components on plastic films, and the virtually unlimited possibilities for variation and the unlimited availability of organic chemistry.
One possibility for the realization of an organic solar cell that has already been proposed in the literature consists in a pin diode [Martin Pfeiffer, “Controlled doping of organic vacuum deposited dye layers: basics and applications,” PhD thesis TU-Dresden, 1999] having the following layer construction:
0. carrier, substrate,
1. bottom contact, normally transparent,
2. p-layer(s),
3. i-layer(s),
4. n-layer(s),
5. top contact.
In this case, n and p denote an n-type and p-type doping, respectively, which lead to an increase in the density of free electrons and holes, respectively, in the thermal equilibrium state. However, it is also possible for the n-layer(s) and p-layer(s) to be at least in part nominally undoped and to have preferably n-conducting and preferably p-conducting properties, respectively, only on account of the material properties (e.g., different mobilities), on account of unknown impurities (e.g., residual residues from the synthesis, decomposition or reaction products during the layer production) or on account of influences of the surroundings (e.g., adjacent layers, indiffusion of metals or other organic materials, gas doping from the surrounding atmosphere). In this sense, layers of this type should primarily be understood as transport layers. By contrast, the designation i-layer denotes a nominally undoped layer (intrinsic layer). In this case, one or a plurality of i-layers can consist either of one material, or of a mixture composed of two materials (so-called interpenetrating networks or bulk heterojunctions; M. Hiramoto, et al., Mol. Cryst. Liq. Cryst., 2006, 444, pp. 33-40). The light incident through the transparent bottom contact generates excitons (bound electron-hole pairs) in the i-layer or in the n-/p-layer. Said excitons can only be separated by very high electric fields or at suitable interfaces. Sufficiently high fields are not available in organic solar cells, with the result that all promising concepts for organic solar cells are based on the separation of excitons at photoactive interfaces. The excitons pass by diffusion to such an active interface, where electrons and holes are separated from one another. In this case, the material which takes up the electrons is designated as acceptor, and the material which takes up the hole is designated as donor. The separating interface can lie between the p-(n-) layer and the i-layer or between two i-layers. In the built-up electric field of the solar cell, the electrons are then transported away to the n-region and the holes to the p-region. Preferably, the transport layers are transparent or largely transparent materials having a large band gap (wide-gap) such as are described, e.g., in WO 2004/083958. In this case, the term wide-gap materials denotes materials whose absorption maximum lies in the wavelength range of <450 nm, and is preferably <400 nm.
Since the light always generates excitons first, and does not yet generate free charge carriers, the diffusion of excitons to the active interface with little recombination plays a critical part in organic solar cells. In order to make a contribution to the photocurrent, it is necessary, therefore, in a good organic solar cell, for the exciton diffusion length to distinctly exceed the typical penetration depth of the light, in order that the predominant part of the light can be utilized. Organic crystals or thin layers that are perfect structurally and with regard to chemical purity do indeed fulfill this criterion. For large-area applications, however, the use of monocrystalline organic materials is not possible and the production of multilayers with sufficient structural perfection is still very difficult to date.
If the i-layer is a mixed layer, then the task of light absorption is undertaken by either only one of the components or else both. The advantage of mixed layers is that the excitons generated only have to cover a very short path until they reach a domain boundary, where they are separated. The electrons and holes are respectively transported away separately in the respective materials. Since the materials are in contact everywhere with one another in the mixed layer, what is crucial in the case of this concept is that the separated charges have a long lifetime on the respective material and closed percolation paths for both types of charge carriers toward the respective contact are present from every location.
U.S. Pat. No. 5,093,698 discloses the doping of organic materials. By admixing an acceptor-like or donor-like doping substance, the equilibrium charge carrier concentration in the layer is increased and the conductivity is increased. According to U.S. Pat. No. 5,093,698, the doped layers are used as injection layers at the interface with respect to the contact materials in electroluminescent components. Similar doping approaches are analogously expedient for solar cells as well.
The literature discloses various possibilities for realization for the photoactive i-layer. Thus, the latter can be a double layer (EP 0000829) or a mixed layer (Hiramoto, Appl. Phys. Lett. 58, 1062 (1991)). A combination of double and mixed layers is also known (Hiramoto, Appl. Phys. Lett. 58, 1062 (1991); U.S. Pat. No. 6,559,375). It is likewise known that the mixing ratio differs in different regions of the mixed layer (U.S. 2005/0110005), or the mixing ratio has a gradient.
Furthermore, the construction and the formation of tandem and multiple solar cells are known from the literature (Hiramoto, Chem. Lett., 1990, 327 (1990); DE 10 2004 014 046). Specifically, the construction and the function of the recombination region in the tandem cells is also described in DE 10 2004 014 046.
Furthermore, the literature discloses organic pin-tandem cells (DE 10 2004 014 046): the structure of such a tandem cell consists of two pin single cells, wherein the layer sequence “pin” describes the succession of a p-doped layer system, an undoped photoactive layer system and an n-doped layer system. The doped layer systems preferably consist of transparent materials, so-called wide-gap materials/layers, and in this case they can also be partly or wholly undoped or else have in a location-dependent manner different doping concentrations or have a continuous gradient in the doping concentration. Especially even very lightly doped or highly doped regions in the boundary region at the electrodes, in the boundary region with respect to some other doped or undoped transport layer, in the boundary region with respect to the active layers or in the case of tandem or multiple cells in the boundary region with respect to the adjacent pin- or nip-subcell, i.e., in the region of the recombination zone, are possible. Any desired combination of all these features is also possible. Of course, such a tandem cell can also be a so-called inverted structure (e.g., nip-tandem cell). All these possible forms of realization for tandem cells are designated by the term pin-tandem cells hereinafter.
Within the meaning of the present invention, small molecules are understood to be non-polymeric organic molecules having monodisperse molar masses of between 100 and 2000 which are present in a solid phase under normal pressure (air pressure of the atmosphere surrounding us) and at room temperature. In particular, these small molecules can also be photoactive, wherein photoactive is understood to mean that the molecules change their charge state under light incidence.
The problem of organic solar cells at the present time is that even the highest efficiencies of 7-8% achieved hitherto in the laboratory are still too low. For most applications, especially large-area applications, an efficiency of approximately 10% is deemed to be necessary. On account of the poorer transport properties of organic semiconductors (in comparison with inorganic semiconductors) and the associated limited layer thicknesses of the absorbers that can be used in organic solar cells, it is generally assumed that such efficiencies can most likely be realized with the aid of tandem cells (Tayebeh Ameri, et al., Organic tandem solar cells: A review, Energy Environ. Sci., 2009, 2, 347-363; DE 10 2004 014 046). Especially efficiencies of up to 15% will in the future probably only be possible with the aid of tandem cells.
The disadvantage of the previously known tandem cells is that only one transport layer system of one charge carrier type is used between two photoactive layer systems (also designated hereinafter as absorber systems or absorber layer systems). The disadvantage in this respect is that this one transport layer system has to be energetically well adapted to an absorber system (in order to enable an efficient extraction of the charge carriers from the absorber system and, e.g., not to form an energy barrier for the charge carriers) and, on the other hand, is intended to be transparent as well as possible (in order not to form parasitic absorption) and is intended to have the best possible charge carrier transport properties. The previously known materials normally fulfill only one of these properties actually satisfactorily and use is often made of a compromise material which fulfills both properties reasonably well to satisfactorily. If a thick transport layer system is required for optical reasons, there is additionally often the problem that the material used does not have a sufficient conductivity and, consequently, the component is limited in terms of its properties (efficiency, filling factor, voltage) on account of the series resistance formed. In some instances, this problem is attempted to be solved by choosing a higher doping concentration. However, these attempted solutions have only a limited success and, as a result of the higher use of dopant materials, the production process for the components is made more expensive, which impedes commercial utilization.
The use of two different transport layer systems instead of the one transport layer system used previously is of great technical importance. The entire transport layer region between the photoactive layer systems has to fulfill a number of functions, namely to achieve a good to as far as possible virtually perfect energetic adaptation, to have as far as possible very good transport properties, to be as far as possible completely transparent and, furthermore, also to be thermally and mechanically stable. Perfectly combining all these properties in only one material for one charge carrier type is virtually impossible. In the previous prior development of organic solar cells, this problem has evidently not yet become clear, since the previous solar cells have been able to be realized with an efficiency of 6-8% even with a material which does not fulfill all properties well to very well. For commercial utilization, however, efficiencies of 10-12% and beyond are absolutely necessary. In order to achieve these high efficiencies, all loss mechanisms within the solar cell structure must be eliminated. One very important building block in this regard is that the transport layer region between the photoactive layer systems in tandem or multiple cells virtually perfectly satisfies the requirements imposed. As explained above, this is practically and technically achievable only if at least two different transport layer systems of the same charge carrier type are used.